Cape Town - Proteas bowling coach Allan Donald says he is worried about the lack of depth in the South African bowling line-up.According to the ICC's rankings for Test bowlers, South Africa's Dale Steyn and Morné Morkel are the most potent opening bowling combination in the world.However, Donald is worried about who will replace them once they get injured. And the former Proteas opener believes it's just a matter of time before one of them, or both, pick up an injury. "That is just the reality of a Test bowler," Donald told Die Burger newspaper.Donald believes the Proteas will struggle to replace the duo if it should happen that both fall foul to injury simultaneously."We would need a good number of bowlers that are ready and capable of taking their places," said Donald.Donald feels South Africa should take note of how England develop their bowlers. England are currently the No 1 ranked Test side, and possess a wealth of depth in their fast bowling department. Doanald is well accustomed with how the English system works as he was their bowling consultant in 2007.He says they look after their bowlers well from u-19 level. "Just look how well they have looked after players like (Graeme) Onions, (Chris) Tremlett, Steven Finn, and (Tim) Bresnan - and prepared them for Test cricket," said Donald.The Proteas have a busy summer schedule ahead before they take on the high-flying English in an away series next winter. This summer they have home series' against Australia, Sri Lanka and the West Indies, as well as an away series against New Zealand.And Donald is worried about the toll those series' could take on his two kingpin bowlers."We will definitely need to give Dale and Morné some time off to keep them fit and ready for the tough series against England," said Donald.